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Recently-Formed Impact Crater
Scientists using the Mars Orbiter Camera on NASA's Mars Global Surveyor spacecraft have discovered a crater that appears to have formed on Mars in the past 20 or so Earth years, and have used it and several other similar craters to estimate the present cratering rate on Mars.
One of the basic tenets of planetary geology is that impact craters have accumulated on planetary surfaces at roughly a constant rate since the early history of the solar system. This appears to have been the case for small craters on the surface of the Moon, as shown by measurements of the length of time that lunar rocks created by small impacts have been exposed to cosmic rays, as determined by laboratory measurements of samples returned to Earth by the Apollo astronauts. This principle should permit the number of craters found on a planetary surface to be used to determine the age of that surface, if the rate at which new craters form is known.
Scientists have previously estimated the cratering rate of Mars by scaling the lunar cratering rate based on the proximity of Mars to the asteroid belt, and by performing calculations based on orbital mechanics. Another way to establish the cratering rate of Mars would be to use long-term observations, say, from orbiting spacecraft, to actually locate new craters.
The new crater is located on the southern rim of the summit crater, or caldera, of the intermediate-sized martian volcano, Ulysses Patera. The site was imaged by the Viking 2 orbiter in 1976 (left, an enlarged portion of the image) and in narrow-angle views by the Mars Orbiter Camera in 1999 (center) and 2005 (right). The new crater, about 25 meters (82 feet) across, is marked by a distinct dark, rayed pattern of ejected material, or ejecta, which is seen to have faded somewhat between 1999 and 2005.
Ulysses Patera, a volcanic shield about 100 kilometers (62 miles) in diameter volcanic shield, located near 2.5 degrees north latitude, 121.3 degrees west longitude, is one of the Tharsis volcanoes and is partly buried by younger lava flows. The summit caldera is about 55 kilometers (34 miles) in diameter.
Image credit: NASA/JPL/MSSS/USGS
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Repeated Clouds over Arsia Mons
Three wide angle views taken by the Mars Orbiter Camera on NASA's Mars Global Surveyor at intervals approximately one Mars year apart show similar spiral dust clouds over a volcano named Arsia Mons. The upper-left image was taken on June 19, 2001, the first day of southern winter on Mars. The upper-center image was taken on April 24, 2003, in late southern autumn on Mars. The upper-right image was taken on Feb. 25, 2005, slightly earlier in late southern autumn on Mars.
Some parts of Mars experience weather phenomena that repeat each year at about the same time. In some regions, the repeated event may be a dust storm that appears every year, like clockwork, in such a way that we can only wish the weather were so predictable on Earth. One of the repeated weather phenomena occurs each year near the start of southern winter over Arsia Mons, which is located near 9 degrees south latitude, 121 degrees west longitude. Just before southern winter begins, sunlight warms the air on the slopes of the volcano. This air rises, bringing small amounts of dust with it. Eventually, the rising air converges over the volcano's caldera, the large, circular depression at its summit. The fine sediment blown up from the volcano's slopes coalesces into a spiraling cloud of dust that is thick enough to actually observe from orbit.
The spiral dust cloud over Arsia Mons repeats each year, but observations and computer calculations indicate it can only form during a short period of time each year. Similar spiral clouds have not been seen over the other large Tharsis volcanoes, but other types of clouds have been seen.
The spiral dust cloud over Arsia Mons can tower 15 to 30 kilometers (9 to 19 miles) above the volcano. The white and bluish areas in the images are thin clouds of water ice. In the 2005 case, more water ice was present than in the previous years at the time the pictures were obtained. For scale, the caldera of Arsia Mons is about 110 kilometers (68 miles) across, and the summit of the volcano stands about 10 kilometers (6 miles) above its surrounding plains.
The Mars Orbiter Camera was built and is operated by Malin Space Science Systems, San Diego, Calif. Mars Global Surveyor left Earth on Nov. 7, 1996, and began orbiting Mars on Sept. 12, 1997. JPL, a division of the California Institute of Technology, Pasadena, manages Mars Global Surveyor for NASA's Science Mission Directorate, Washington.
Image credit: NASA/JPL/MSSS
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